Huh, I use the 3.5.1 version (great classic UI too) and am satisfied with it, but it does tell me to upgrade.Malwarebytes Anti-Malware 1.75 (this is best light version over 2.0 version and above)
I also tried the Portable/Bootable version available on here and it's v2.00 something.
https://gbatemp.net/threads/unofficial-malwarebytes-bootable.481046/
Don't know why MBAM downgraded their UI. Seems like everyone does for simplicity/minimalism.
E.g.:
Much as I dislike these logos, I got used to them since it's what they use.