My personal take. This fight will happen since this is a big fight and should be very easy for Spence. Spence should be able to stop this past-prime version of Thurman or at least win an easy unanimous decision if he wants to stay safe.
At first glance I see Thurman continuing to practice to anticipate and hone his skills, if Thurman & Spence officially box in the ring in the future, Thurman understands quite well some of Errol Spence's attacks on his opponent.
I remember that at that time Thurman had refused Spence's offer, but if you look at the events that have happened between the two of them, it is clear whether this boxing is official or not depends on Errol Spence's decision, if he hadn't made fun of or held a grudge against Thurman a while ago.
Some experts and boxing experts predict, if Errol Spence forgets what happened between the two and Errol Spence doesn't hold grudges and is selfish in this boxing, of course he will get the best ticket to fight Thurman in the future, that's all if Spence doesn't change his mind.
I know that prime Thurman is capable of beating Spence. We'll see mate if Thurman's fire returns to its former glory days. He isn't that old but he needs hunger and motivation in order to beat Spence.
I heard the rumors before but I haven't paid much attention. But I did follow Thurman a lot during championship days. Thurman from the past was so hungry, eager, and determined. I actually like him when he was being honest and verbally threw offensive remarks to his own handlers (PBC and Showtime) and to Floyd Mayweather jr. who kept the belts hostage from mandatories, ranked 1 contenders, and Thurman as then the WBA regular champion. Thurman was obviously mad and disgruntled as to why he isn't getting a shot at the real belts. I am not even sure if Spence's challenge was for real and at that time he ain't a champion either so Thurman won't get anything if he wins. And Thurman was right, if you want to be the best then why wasn't Spence challenging Mayweather at that time.
This is the same scenario right now, overdue title shots for the next generation of prospects; WBA regular champion Eimantas Stanionis and top 1 ranked contenders Vergil Ortiz and Boots Ennis. The WBA now mandated both Stanionis and Ortiz to face each other and the winner gets the Super WBA belt held by Spence. I doubt it happens though, why would Stanionis face a mandatory when he is already the regular champ, he should get Spence right away. The same situation for Ortiz, he gets nothing fighting Stanionis, it's a fake belt. I don't think Spence would want to fight these young undefeated contenders that are too risky, he rather move up and try to become a 2 division champion for his legacy.