Modrinth uses Cloudflare to protect against Distributed Denial of Service (DDoS) attacks. Cloudflare acts as a shield for websites. It filters incoming traffic before sending it to the actual service. Because of this, many websites, including Modrinth, share a limited pool of IP addresses.
When illegal websites are banned in Ukraine, both their domain names and associated IP addresses are blocked. Unfortunately, some of Modrinth's IP addresses provided by Cloudflare overlap with those of blocked websites.
Most Ukrainian Internet Service Providers (ISPs) use deep packet inspection to allow connections to non-blocked domains. However, some ISPs use simpler blocking methods, which may inadvertently prevent access to Modrinth.
Modrinth does not intentionally block Ukrainian users. If that were the case, you would see a specific message from Cloudflare.
Here's some things you can do if Modrinth does not open for you in Ukraine:
Contact your ISP. Ask them to investigate why Modrinth is being blocked on their network.
Use Cloudflare WARP. It makes your connection opaque to ISP, and may help bypass the restrictions, but will slightly reduce your internet speed.