Dropping 1.8 support in plugins becomes a "must", because it's honestly a waste of time for developers and creators to create projects that support nearly 8 major versions of the game. At least 90% of people on Bukkit, Spigot and Paper do this all as a hobby, and to improve their programming skills, there's no reason for them to hinder future updates by trying to ensure backwards compatibility for a very small minority. As someone who's ran a ton of projects since 1.9 came out, trust me when I say, if you don't have a PvP oriented gamemode, it's worth the risk in upgrading. For example, many Prison and SkyBlock servers now run 1.12 (with backwards compatibility), Survival and Creative are on the latest. It's only really Factions and KitPvP (and a few minigames) that are on 1.8, with HCF on 1.7. BungeeCord natively supports 1.8+, so, if you run a network, there's no reason to not run the "best" version for your specific gamemode. The only argument for staying on 1.8, aside from PvP is because of the mods, and a lot of that is due to there being no update in ages (I think 1.8 -> 1.9 was something like 2 years). But, with the developments to Forge and Fabric, and with Mojang releasing the mappings, it's a fantastic step in the right direction for the modding community, and that should see some of the popular 1.8 mods updated/remade on future versions. Plus, Mojang is releasing combat changes and they are trying to find a middleground (granted, they want parity with Bedrock for whatever reason), and they are taking a ton of user feedback. Combine this with the modding advancements, and, soon, there'll be no reason to be on legacy versions.