The last Ubuntu-MATE for i386 was Ubuntu-MATE 18.04 LTS. Being a flavor it had 3 years of supported life.
Two other flavors of Ubuntu (Lubuntu & Xubuntu) offered a i386 (32-bit x86) release of 18.10, but the building of i386 ISOs was finally stopped in the disco development stage for those two flavors (ie. 19.04), though any installs of Lubuntu 19.04/Xubuntu 19.04 received upgrades for the life of the product if used for an install.
You didn't specifically state which 32-bit architecture you're wanting; as i386 or x86 32-bit is only one 32-bit architecture. Ubuntu MATE 32-bit also provided support for armhf or 32-bit ARM; refer Raspberry Pi | Ubuntu MATE if you're not after a specific i386 32-bit machine.
Ubuntu 18.04 LTS is still supported (standard support ends in April 2023) so a 18.04 install (of i386) will get security fixes for packages from 'main', but that's only a portion of the system.
I'm still a user of i386 or 32-bit x86 hardware (on occasion anyway) and personally I'd recommend Debian (note: I've only used up to Debian 11 on 32-bit x86 or i386 hardware (Debian 12 is actually close to release, but I've felt no desire to test it on i386))
You should specify what 32-bit architecture you're asking about though, as that really matters. 32-bit ARM is still supported !