I was thinking about downgrade my server to 1.8 because many players told it's pvp easier (also hacking easier). And it's not causing FPS / lag spike like 1.10 1.11 for my players. But one thing I worried about is not many plugins stable and if it have error , author won't fix it .
1.9+ do not cause lag spikes that 1.8 does not cause. There is no need to downgrade in order to get the 1.8 PVP mechanics; you can use a plugin. You are correct, most authors - and Spigot itself! - do not support 1.8.x anymore. Bugfixes, including those that are relevant if it comes to security, will most likely not get fixed.
pvp mechanics is beside , most of my player trying to join my server with 1.8 version because they have low end computer . That's the problems
This may be a good video to watch I find newer versions actually perform better, however, doing what I did in that video will still allow 1.8 clients to join, while your server is on a safe and updated version.
Minecraft is a very lightweight game. You'd have to be using a fairly bad computer to often have problems with the game. You can't downgrade to an old version just for a select few. There are other ways in which you could try to make it easier on those computers, removing entities for example and advising them to use client mods like Optifine. Old versions won't be updated, neither by Minecraft, nor Spigot, nor plugin authors. You're missing out on lots of bug and security fixes as they come. I wouldn't recommend 1.8 in production unless you keep it up to date yourself with any developments, and your plugins are also kept up to date for security/bug issues. I think that's what larger networks do - just use an old version and bring it forward making any necessary patches they need to.
i never expected minecraft is a lightweight game . has playing it since 2010 its not better since that too much p/s : my computer is not low-end
Minecraft is NOT a lightweight game. It has low requirements, but it is not at all lightweight. You won't need much to run it, but you'll still need a high end PC in order to run it perfectly.
Your question is kinda strange. The title asks if 1.8 is a stable version? Yes it is quite stable (all missing security updates aside). But then you asked if your plugins will be ok - the answer is it depends on the way the plugin is coded. If it utilises features that weren't changed since 1.8 it should be ok, but if it does (and because of sound changes many plugins had problems) it depends on how the plugin is coded and if it supports backwards compability. So you just can't get a good enough answer without telling us every plugin you have. That being said, my advise is don't downgrade.