Addon authors support their addons and perhaps they could answer your questions as to why it is no longer available. I can't. My guess is their product had such low usage numbers they stopped supporting it on Thunderbird, but honestly if you want a definitive answer ask the folks that write the addons Looking at their web site they consider their product and addon for a web browser https://getadblock.com/en/ as Thunderbird is a mail client and basically has few browser features outside of reading RSS feeds I guess they chose to drop Thunderbird as there is limited browser like interactions to block adds in.