i don't think theres anything "wrong" with still sharing a bathtub with an 8 year old but that does sound like a good time/age to move on from that particular father-son activity.
Like, unless your son's specifically asking you to hop into the tub with him, I don't think it'd be wrong to phase it out.
Like, again, I don't think it'd be wrong or weird not to, but I don't see the benefit of it any longer and it's definitely a "small child" activity to bathe with your dad and it might be something your son could get made fun of, sadly.
It's like - there's no reason to _stop_ drinking from a bottle, or to stop using a pacifier from time to time, at a certain point it's simply not a necessity anymore and the "weird" optics outweigh the benefits.
There's other things that I think are very valuable to parent child bonding, like them being allowed in your bed if they're scared or sad - even when they _could_ be sleeping in their own bed.
That's something where telling the child it might be "too old for that kind of stuff" feels much more harmful.
But you do you OP.
And wow at the weirdos who think sharing a bathtub is a "weird" thing to do. For small children, the tub is _literally_ a tiny water park. Playing with them while caring for their personal hygene is practical and super fun for the kid.