SolarWinds does support MS SQL Server 2008 R2 SP3. Says it right at the top of the link you mentioned.
I used to use that same SQL version and still have one overseas that does. In new versions going forward, I believe post NPM 12.1 (will fact check that in a bit), they're not supporting it anymore. Can't recall off the top of my head if it was both 2008 server and SQL 2008, or just one or the other.... It's in the release notes though.
Whoops! I noticed it was listed at the top of that document, but since they didn't list any versions by it I was being a little paranoid and wanted to ask just in case. Looking at the release notes like you mentioned, we should be good until we upgrade to 12.1 or forward.
Thanks for answering a silly question!