完璧じゃない気もするけど一応これで良さそう

RewriteRule ^(.*?)(?:_yyyyMM)?(\..*?)$ https://280blocker.net/files/$1_%{TIME_YEAR}%{TIME_MON}$2 [R,L]

https://280b.web.fc2.com/280blocker_adblock.txt
https://280b.web.fc2.com/280blocker_1blocker.1blockpkg
https://280b.web.fc2.com/280blocker_domain.txt
https://280b.web.fc2.com/280blocker_domain.zip
https://280b.web.fc2.com/280blocker_domain_ag.txt
https://280b.web.fc2.com/280blocker_adblock_yyyyMM.txt
https://280b.web.fc2.com/280blocker_1blocker_yyyyMM.1blockpkg
https://280b.web.fc2.com/280blocker_domain_yyyyMM.txt
https://280b.web.fc2.com/280blocker_domain_yyyyMM.zip
https://280b.web.fc2.com/280blocker_domain_ag_yyyyMM.txt