The midfielder will pick up around £700,000 per week in Dammam, where Anfield icon Steven Gerrard is currently the team's manager.
Henderson verbally agreed to join Al Ettifaq two weeks ago, but wanted to speak with Jurgen Klopp before making a start on any official paperwork.
Liverpool will receive an initial £12 million for their captain, a fee which could rise to almost £20 million - admittedly a decent chunk of change for an ageing player.
Let's talk about the headline
Homosexuality is illegal in the Saudi state, which means people can be imprisoned - and even sentenced to death in extreme cases - for simply being themselves.
Kop Outs said in a statement: "What was it about the multi-million-pound contract that first drew you away from your commitment to human rights?
"Given choices (Henderson) has recently made, Kop Outs doubt and question if he was ever an actual ally.
"We are deeply disappointed that he is choosing to work as part of a sports-washing operation, attempting to distract from a regime where women and LGBT+ people are oppressed, and that regularly tops the world death sentence table."
The statement continued: "It is gobsmacking that a father of young children is moving to a regime that is set to break its promise on executing juveniles.
"(Steven) Gerrard's participation in this sports-washing is particularly galling, given his previous support for LA Galaxy teammate Robbie Rogers.
"We hope that Henderson and Gerrard don't have to personally experience the harsh reality faced by LGBT+ people and real advocates for human rights.
"We question the decision of LFC to accept money from such a regime. We have long campaigned for Liverpool to resist investment from countries with poor human rights records."
Henderson, an England international with 77 caps under his belt, has presented himself as a vocal ally of the LGBT+ community throughout the years.
By opting to ply his trade in Saudi Arabia he is betraying those he stood for, including the Rainbow Laces campaign he was previously proud to embrace.
As mentioned in Kop Outs' statement above, Gerrard is not blameless - and neither are Roberto Firmino and Fabinho, who have both accepted Saudi deals this summer - but Henderson has previously gone the extra mile to speak out as an ally to LGBT+ people, so he will be treated differently.
Even if his beliefs haven't changed, he'll now have to keep them private or risk upsetting those who write his cheques - at least that is what can be surmised from Al Ettifaq's announcement video, which greyed out rainbow armbands Henderson has worn.
What does Saudi Arabia get out of it?
By signing some of the world's most notable football players and managers to its domestic league, the Saudi state has been accused of sports-washing its appalling human rights record.
In addition to outlawing homosexuality, the Gulf state also executed 81 people in just a 24-hour period last year and assassinated dissident journalist Jamal Khashoggi in 2018.
By making the switch, Henderson is allowing his name to be used in an apparent attempt to sanitise the Saudi image, even if official lines would argue recent investment in football is to enrich their position in sport on an international stage. Anyone familiar with LIV Golf will understand this point even more deeply.
"The history of new football leagues is mixed to say the least," the aforementioned statement by Kop Outs continued, casting doubt over the Saudi Pro League's long-term future.
"Wages unpaid or late (11 out of 16 Chinese clubs in arrears as of November 2021), transfer fees unpaid for purchases (Al-Nassr still owe Leicester City for the 2018 signing of Ahmed Musa), and most disturbingly the plight of Zahir Belounis, who was trapped in Qatar for two years when his club refused to grant him an exit visa.
"We urge football, home and abroad, to be a force for decency in our world. For football to be the beautiful game, it has to confront the ugly, and not try to be a fig leaf for death, torture and abuse."
What about Henderson?
Al-Ettifaq will pay Henderson somewhere in the region of £700,000 per week, which is about £36 million per year, a vomit-inducing amount of money.
He was part of Jurgen Klopp's plans for the new season, even if in a reduced role, so his decision to leave Liverpool so abruptly can be viewed in a negative light.
The veteran midfielder built his name at Anfield - in the Premier League, renowned across Europe - and was earning a pretty penny doing so.
Liverpool paid Henderson around £125,000 per week and there were still two years remaining on his contract.
Admittedly, he will get more regular first-team action in Saudi Arabia than at Anfield, but that is certainly not the reason he'll play in the Saudi Pro League next season.
Even in his twilight years, Henderson would have been able to walk into many teams in a number of high-level leagues, such as LaLiga, Bundesliga, Serie A and even America's MLS.
The midfielder will want to continue representing England, but playing for Al-Ettifaq is unlikely to instil much faith in manager Gareth Southgate, who held meetings with the player before his move.
Still a club legend just tarnished
It should be clarified that nothing can take away from Henderson what he has given to Liverpool over the last 12 years and what he has achieved in his career.
The midfielder is a bona fide club legend and will be remembered as such but his decision to move to Saudi Arabia will also be etched into his folklore.
Henderson joined Liverpool in 2011, was made captain in 2015 upon Gerrard's departure, and won every trophy possible during his stay.
It has been a glorious 12-year ride at Anfield for the player but what may be his final career move raises questions about his legacy.