This was already discussed before, last time in topic #19378 I believe, but the discussion fizzled out; the subject came up on the Discord again and frankly I still think that stipulation doesn't really make sense and is pretty counter-intuitive.
Why are solo and multiple_views mutually exclusive with one another?
No real reason for it was given other than "it's like this because it is and people said that when the tag was made" or "I like the way it is", but in practice it makes it essentially impossible to search for all the art of just a singular character, a concept that frankly shouldn't be impossible to search for and that sounds like what the solo tag should be for on paper.
- solo focus does not solve the issue because solo focus is explicitly for "more than one character in the picture, but one is overwhelmingly the focus".
- chartags:1 is effectively useless for this purpose now for most characters featured in gachas (or even any video game with named skins) since each skin gets a chartag of its own, and that would exclude all those results from the search.
- ~solo ~multiple_views isn't really a good equivalent because multiple_views can include more than just the one character you're looking for - eg. post #3921065 showing up in an astolfo_(fate) multiple_views search.
- You can probably search for this with a strange, convoluted search with a ton of exclusions like astolfo_(fate) 1boy -1girl -multiple_girls -1other -multiple_others possibly maybe?
Basically, it's impossible as things are to search for a concept as straightforward as "all the art of a single character and only this character" without an extra convoluted search and/or false positives in the search.
solo should include multiple_views of one singular character (such as post #4750933), because that still falls under the definition of what solo sounds like it should be, and "only one view of one character" could still be searched as a result - either with solo -multiple_views, or with a tag of its own.
As far as concerns of diluting the solo tag go, it has 3.1M posts currently, compared to multiple_views's 64k, that's barely 2% of the solo count - and it can be excludedfrom the search, as opposed to the reverse search (all art of a single character) being essentially impossible to search for as things currently are.