I voted to close the question as a duplicate with a similar but not exactly the same question ( Sitecore 9.0 works with solr 7.2.1? ). But the answer stays the same, just with changes to the versions:
Solr 8.X is not supported by Sitecore 9.X officially.
But does it mean Sitecore 9.X will not work with Solr 8.X? No, it doesn't. It means that Sitecore 9.1 is not officially tested against that version. And it means that if you will use Solr 8.X with Sitecore 9.X and you experience any issues, Sitecore Support will be allowed to tell you "Sorry, but you're using unsupported version of Solr".
I know about plenty of projects running on newer Solr versions than are supported officially and they do work well.
So should you use 8.X or install 7.2.1? One good answer does not exist.
For the reference: Solr compatibility table