I am sticking with High Sierra 10.13.6 on my older iMac. Is there a current VPN service that can now, in 2021, be used with it

I've been using NordVPN for years with no problems (five Macs) so at least one user in a row has no issues. If you need a VPN (my preretirement company required it for certain evolutions) most have a trial period.

If you then decide to get rid of it, free AppCleaner will remove it completely.

There are many interesting VPN articles on the web; suggest you do some research there too.

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they are useless from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Former Malware Distributor Kape Technologies Now Owns ExpressVPN, CyberGhost, Private Internet Access, Zenmate, and a Collection of VPN “Review” Websites

Welcome to the Apple Support Communities.

No VPN is ever needed on a Mac. They just cause issues.

Jack