There is nothing illegal about using a VPN in Ukraine, although domestic VPN services may be required to keep logs that the Ukrainian government can request.
However, such requirements don’t apply to international VPN services. Proton VPN is based in Switzerland. Under Swiss law, we don’t have to save any user connection logs, nor can we be forced to perform targeted logging on specific users. So your private browsing history stays private and can’t be turned over to a third party.