VIaBackwards, 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 1.9) to newer servers.
See ViaRewind for 1.7/1.8 clients support.
This is a repost of https://viaversion.com/backwards (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May be outdated (manually re-uploaded).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