These page list all the supported Slackware and MATE version:

Slackware 15.0:

• MATE 1.28
• MATE 1.26

Slackware 14.2:

• MATE 1.18
• MATE 1.16
• MATE 1.14

Slackware 14.1:

• MATE 1.12
• MATE 1.10
• MATE 1.8
• MATE 1.6

Slackware 14.0:

• MATE 1.6

Not affiliated with or endorsed by Slackware Linux, Patrick J. Volkerding, or the SlackBuilds.org project. Slackware is a registered trademark of Patrick J. Volkerding and Linux is a registered trademark of Linus Torvalds. Use at your own risk, YMMV, and all that.