I’m still using my SE1, on the most current iOS, and it works great. I haven’t noticed any slow down from when it was new, but my battery life did deteriorate. I need to replace the battery, but in the meantime I recently switched to another brand new SE1 that I was keeping as a backup. Battery life and performance is perfectly fine.
True one-handed use is important to me so I plan on continuing to use the SE1 even if it stays on iOS 14, since Apple will likely continue to release security updates for truly serious threats, until it is no longer functional or fixable for whatever reason, or if there is a new feature(s) that I consider must have.