No if anything newer OS is faster. Apple works extremely hard to improve battery life and has been gradually reducing the size of the batteries in their laptops to get the weight down. My latest Mac has a 50Wh battery that lasts 18 hours in real world use. Years ago I used to get 5 hours from a 100Wh battery.

Those battery improvements aren’t just from efficient hardware - it also comes from more efficient software.

If you still had a HDD that’d be a problem, the software assumes a fast drive now and leaves memory on the disk that, years ago, would have been pulled into RAM, but you won’t have that problem since you have an SSD (also… that’s a big part of why Macs don’t need much RAM compared to Linux or Windows).

Apple drops support for old Macs when the cheapest configuration of that Mac is no-longer fast enough. With upgrades, like yours has, it’s often fine.

… but seriously when you can afford it an M1 processor will be an order of magnitude faster. I’ve got a 2014 Mac Mini at home and also an M1 laptop. The speed difference is astronomical. Mostly I just keep the Mac Mini around for backups/etc since connecting a laptop into an external backup drive is annoying.