They solved a real problem of having parts of the page that changes and other parts that didn’t. It’s a shame they were deprecated without a modern replacement.
An "include" mechanism (like what html provides) should have been proposed to replace this use case.
The deprecation of frames forced dynamic websites (wether it is client or server side) on everyone and i feel like it is in part responsible of why people stopped to handcraft websites.
Isn't iframe the replacement? iframe is not deprecated.
There would have been many better separators... but good idea to bring formatting into it as well...