We're partially there, under Storage is an option allowing you to set how long to keep messages and I've set mine to one year. Possible: forever (default), 1y, 6mo, 30d - and it works, my old chat messages (not the whole chat, just individuals) are properly culled over time.

Edit: in context, Google Messages has none of these features and I have friends still married to Google Voice who send me tons of pics. Culling SMS requires using a third party tool to export and re-import etc. leagues behind Signal. None of it's backed up without the same third party tools as well and no built in image management.