ViaRewind, addon of ViaVersion plugin (https://viaversion.com) and ViaFabric mod (https://www.curseforge.com/minecraft/mc-mods/viafabric), allows you to connect with older clients (down to 1.7.2) to newer servers.
You need also ViaBackwards for 1.10+ servers.
This is a repost of https://viaversion.com/rewind (under GPL license), making easier to install it with CurseForge launcher.
May be outdated (manually re-uploaded). The JAR is the same from the upstream URL.
The JAR can be used on Bukkit, BungeeCord, Fabric, Sponge and Velocity.
Discord: https://viaversion.com/discord