Since we moved Wikimedia messages questions to Phabricator, most users got used to using it. Moreover, the main benefit in having MediaWiki support requests on translatewiki.net was the ability for Siebrand to go through them in batches, but AFAICT the WMF has since stopped systematically taking care of its own i18n issues.
From my export of https://translatewiki.net/wiki/Category:Open_support_requests_for_MediaWiki, the involved users are the following (99.5 % reports and 75 % reports covered):
$ grep ThreadAuthor translatewiki.net-20151204222443.xml | sort | uniq -c | sort -nr | sed 's,<ThreadAuthor>,@,g' | sed 's,</ThreadAuthor>,,g'
98 @Purodha
42 @Macofe
10 @Base
8 @Lokal_Profil
8 @Liuxinyu970226
7 @siebrand
3 @Kghbln
2 @Pikne
2 @Milicevic01
2 @liangent
2 @jeblad
1 @Withoutaname
1 @veeven
1 @Troubledasset
1 @아라
1 @The Evil IP address
1 @Tgr
1 @Stam.nikos
1 @Srtxg
1 @Shirayuki
1 @Protnet
1 @Paladox
1 @Nikerabbit
1 @Nemo_bis
1 @Metalhead64
1 @McDutchie
1 @Mattflaschen
1 @Marmase
1 @Lloffiwr
1 @Kolega2357
1 @Kaganer
1 @Gleki
1 @Dude1717
1 @Calak
1 @Beta16
1 @Amire80
1 @Ainali