Addon Manager still has immature parts. It was only in 5.2 that it was implemented to replace the Preferences-based addon management. Other than some tweaks in the 5.2.0 beta cycle, it hasn’t had any enhancing.
Requests for a feedback mechanism for confirming connection ( or failed connection) were submitted during the beta cycle. It took so long for beta testers to understand the subtlties and set up tests that the submissions were too late in the cycle.
There are a significant number of issues filed that were related to the Addon Manager. But since the code is not yet documented, @Nick-Hall is the only person picking up tasks for it.
The localization fallback strategy for addon lists in JSON are one of the items that is unknown. (Is addons-en.json the base? There’s addons-en_GB.json too. And I vaguely recall a language agnostic addons.txt before we switched to JSONs.)
@dsblank recollected having been involved in the decisions about the original addons-xx.txt server path. Maybe he has some opinions about improving the feedback?