I prefer all-original as much as possible, but I’m also a realist.
I actually also prefer an acoustic guitar strap to be tied to the headstock vs the back of neck, in terms of balance and feel.
However, if I get a vintage guitar that has a strap button, I simply use it- like my ‘66 Gibson J50. In this case, it actually looks like it was probably added in the ‘60s-‘70s, so this button is now “vintage”, and part of the guitar’s story. My particular guitar also has replaced tuners (exact replicas, so not a huge deal), and what appears to be a very well matched plugged jack hole in the lower side.
What bugs me are the irreversible mods. Electronics, jacks, and pins can be reversed. But the most common mods on vintage guitars that I’ve seen are bridge replacements and tuners. More often than not, Gibsons of the ‘60s have the ADJ bridges removed entirely, which is odd since that is one of the biggest aspects of their 1960s sound IMO. Guilds often have bridges replaced without the buyers or sellers even being aware, so that would be the one to watch out for and look closely for. I assume this was usually done during neck resets on guitars that had previously had their bridges shaved.