how can i continue running NordVPN (or similar) on Yosemite osx 10.10.5?

Then upgrade to Mojave. Almost anything that runs on Yosemite will run on Mojave, and a lot of more current software will too. It is only with Catalina and later you have to worry about 64 bit only software.

https://discussions.apple.com/docs/DOC-13571

I'd also add any Mac that can run Yosemite, can upgrade to El Capitan.

A lot of that software is hard to find because the App Store only keeps more current software versions. What's your purpose for using a VPN? If you want to block anyone from seeing you, most browsers have a private window.

See the browsers on this page:

https://discussions.apple.com/docs/DOC-250003586

that are compatible with Yosemite.

Be careful when shopping around for old software, as optimizers can actually worsen the performance of your Mac.

VPN can be a double edged sword. While it might give you access to networks that are generally not available inside your country, access to other networks within your country may be restricted while you use a VPN. This is the case of work VPNs where they control your traffic through their network, and their security limits what websites you can browse. Be aware some people who are musicians who must limit their access on VPN may not be able to afford the licensing to distribute worldwide. You may want to contact the musician and let them know this makes it difficult when sharing their data. They should look for a host that is more open about sharing data.

Start with these articles:

A VPN is intended to give you a secure tunnel to a private network such as your employer or school.

Using one to access content on the Internet gives you a tunnel into the VPN provider. From there on out to the Internet it's no different than not using a VPN, except that you have now give a single entity, the VPN provider access to all of your data traffic and the ability to collect and sell that data. And yes, no matter what their public policy say, they log everything and will happily turn it over to law enforcement.

"Don't use VPN services"

https://gist.github.com/joepie91/5a9909939e6ce7d09e29

"How VPN services resell your data"

https://thebestvpn.com/how-free-vpns-sell-your-data/

Some research into one of the VPN providers:

https://news.ycombinator.com/item?id=17258203

And some of the bugs in older VPN code...

https://mobile.twitter.com/CiPHPerCoder/status/1044625129278443522

Why are you running a VPN to begin with? It is doing absolutely nothing to protect your privacy or secure your data. Exactly the opposite, in fact.