If they are using OpenVPN, decent providers can give you a *.ovpn file for the "router use" or "manual/linux use", and you can import it in the luci-app-openvpn web interface
For example, Mullvad has it https://mullvad.net/en/download/linux/
"Unable to use the app?
If you cannot use the Mullvad VPN app, you can download configuration files to use with the official OpenVPN or WireGuard clients."
ExpressVPN has this (these are steps for Asus routers, but it shows how to get the *.ovpn file you can load in OpenWrt too) https://www.expressvpn.com/support/vpn-setup/manual-config-for-asus-router-with-openvpn/
NordVPN has it https://nordvpn.com/ovpn/
and as I said most others do have this feature where you can download a *.ovpn file with your VPN configuration you can upload to OpenWrt.
This is too little information, you need a full *.ovpn config file, where the VPN service provider has written the OpenVPN configuration of his service.
Keeping an up-to-date list of OpenVPN config files so the user does not need to upload his *.ovpn file requires having an account in all supported VPN providers, which is not free, and also time to update and check and solve any incompatibility problem.