It appears that MalwareBytes 3.5 is the last/final version of MB that will be XP/Vista compatible:

https://forums.malwarebytes.com/topic/191650-malwarebytes-3-frequently-asked-questions/?tab=comments#comment-1243649

Malwarebytes 3.5 is the version that will be provided to Windows XP and Vista users who wish to install Malwarebytes on these legacy operating systems.

Users running Malwarebytes 3.5 on Windows XP and Vista will continue to receive on-going protection updates to keep safe from the latest infections. However these operating systems will no longer receive program upgrades, such as component packages or newer program versions, for new features. Malwarebytes will continue to release bug fixes, stability improvements, and other upgrades for the XP and Vista platforms on an as needed basis.