There are some BBS based mail networks, but they are hyper specific to retro stuff I believe. Based on your description of Usenet I think what you miss is the

There are some BBS based mail networks, but they are hyper specific to retro stuff I believe. Based on your description of Usenet I think what you miss is the context around Usenet (exact period of life, young/single/student for example) and not the Usenet itself. You can find plenty of places where there are forum like discussions, but it won't be the same without the context.