Email has never had explicit threading - email has a bunch of usage habits that many clients interpret as attempted threading (which often results in threads of steam receipts in my gmail webview FYI).
There are a number of clients that are particularly good at respecting the specific approaches to threading used by mailing lists and that come with a plethora of powerful keyboard interactions to make browsing a whole bunch of emails trivial. But newer devs may be quite unfamiliar with those options and I don't think it's particularly easy information to learn... especially if your webmail has "just worked" for most other uses.
There are a number of clients that are particularly good at respecting the specific approaches to threading used by mailing lists and that come with a plethora of powerful keyboard interactions to make browsing a whole bunch of emails trivial. But newer devs may be quite unfamiliar with those options and I don't think it's particularly easy information to learn... especially if your webmail has "just worked" for most other uses.