I think, The vast majority of AbemaTV is region-locked, so you will need a VPN. However, many mahjong broadcasts are not. Like you mentioned, the RTD Leagues, M.League, and other pro leagues can be watched from outside of Japan. It's just a select few that are blocked, likely due to the sponsors and licensing issues. If you want to watch VODs of previous broadcasts, you will need Abema Premium. However, you will still need a VPN for region-locked videos.