Disclaimer (as usual)
This here represents my personal opinion only.
What I write here doesn't mean, that you think the same.
I just want to share my thoughts about this topic to make things clear.
So many servers out there are running 1.8.x or even older versions, which I don't understand.
Sure, there are some reasons, but in my opinion are those straight up stupid and a lazy excuse to not move forward.
"The new 1.9+ combat is bad" argument
This is without a doubt one of the biggest "reasons" of players to stay with 1.8.x.
They won't switch because "The combat update in 1.9 made combat worse."
That's just so stupid. Minecraft never was meant to be a PvP-focused game. And the how I call it "wet dream of a clickskript kiddie", which is 1.8 is not good.
1.9 sure changed combat, but not in a bad way. It added a cooldown, which in my opinion improved the PvP and PvE.
Instead of just mindlessly spamclicking the attack-button, you know have to make each hit count, to deal the most damage and spamclicking will punish you more, because... It's something you shouldn't do in the first place.
Let me ask you a question: If you really stay at 1.8 because of combat, do you not want to get and use new blocks, mobs, etc.?
If you answer "I want new stuff" then you shouldn't stay on 1.8.
You miss so much new stuff like Elytras (1.9), New mobs like the polar bear (1.10), blocks like Observer (1.11) or the concrete blocks (1.12) and don't forget other new mobs like dolphins or fish (1.13).
Each update adds new content to Minecraft and makes it more fun to play.
Staying on 1.8 just won't let you get those new blocks, entities and features.
Also one thing to remember: Each update often also means improvements in terms of code and security.
Only god knows, what for bugs and security issues exist in 1.8, which are fixed in later releases,
You literally make yourself and your server much more easy to be attacked.
Plugins "exist" for 1.8 combat
Since 1.9 was released, do plugins exist, that change the combat back to 1.8.
You now will say "It's not the same" but I have to say you're wrong. The PvP is almost (to 99%) the same like in 1.8.
But again: Having 1.8 combat is already a bad thing, because people with clickskripts or hacks can easly abuse that to destroy the fun of you and others.
"Big servers have 1.8" argument
This is the second most argument I read or hear from others.
Something like "But servers like Hypixel allow 1.8 players to join!"
Yeeeeeah, about that.
First of all: You should stop to make a wanna-be server similar to Hypixel, HiveMC or similar. You will fail and you'll fail badly.
Next: Servers like Hypixel or HiveMC don't use normal plugins and spigot... They most likely have a seperate special jar for their servers to make it possible for easy adding and use of minigames and similar.
The fact that 1.8 players can join the server doesn't mean that it is 1.8. It can easly be a 1.13 server and just allow 1.8 to join.
I mean with plugins like ViaVersion and ViaRewind is it already possible, to allow 1.8 clients to join a 1.9+ server.
You should really stop to take other (big) servers as role model for your own.
You won't get far, if you just become another SkyBlock, PvP and Prison server. Make something unique and stop being a copycat!
Thanks for reading. Feel free to leave a comment, but please be nice and polite since it would only destroy your own reputation, when you just insult me or others.
I don't know what version(s) you play, but I for my part enjoy it, to play 1.13, because new content is always better than just boring PvP.