Like 2021 rev.2 this release is also different from regular updates. It is closely correlated with IANA's 2021d and 2021e. See below for more details.
TZDB version 2021c reverted most (but not all) time zone merges from 2021b. As Android has not applied any of them and there were no other rules update, it is a no-op for Android.
Then TZDB version 2021d was released with Fiji changes. That corresponds to ICU‘s 2021a2. But 2021a2 also picked up a "Link America/Panama America/Coral_Harbour" change from IANA’s TZDB release in the backward file.
ICU's 2021a3 is 2021a2 with Palestine changes. No extra change there.
tzdata2021a.tar.gz in this Android TZDB update is ICU's tzdata2021a3.tar.gz with Link change mention above reverted. Not reverting it will put America/Coral_Harbour under Panama in tzlookup.xml. Right approach here is to update tooling, but that will take some time.
In 2021b several time zones which were alike since 1970 were merged. It received backlash in tz mailing-list. Please check September archive for discussions around the merges.
This contains a subset of the updates from IANA's 2021b update, but not all because they cause problems for various libraries and OSes, Android included. New links cause problems for the tzlookup.xml generation process, which have yet to be resolved. Android API behavior is influenced by ICU, so while ICU are forked and until things become clearer, so is Android.
ICU decided to skip zone merge changes and named their release 2021a1. Unfortunately there are hardcoded places in Android which expect tzdb version to be exactly 5 characters and we release it as 2021a rev. 2.
input_data/iana/tzdata2021a.tar.gz is renamed ICU's 2021a1.tar.gz. As it is not signed, tzdata2021a.tar.gz.asc was deleted for this release only.
2021a1 contains the following changes applied on top of 2021a:
# Portugal observed DST in 1950 https://github.com/eggert/tz/commit/ed2f9d6b01d1256a4d463d2ceb953e15e1673d72 # Fix Guyana LMT and later historical transitions https://github.com/eggert/tz/commit/0252f09cdff3769ecbf6adfdaf6c47055bbfb74b # Add Barbados DST 1942-1944, fix end of LMT https://github.com/eggert/tz/commit/42efb563b8224a20c1a6480b38427e5b4634da36 # Backzone fixes for Gambia, Malawi, Sierra Leone https://github.com/eggert/tz/commit/bdb47c5fd68996c04a69d6b23c4c4a5a81df5c95 # Sierra Leone did not use DST 1957-1962 https://github.com/eggert/tz/commit/6982e34ab92cf13f3f3fb20212aca8aa2e4f0aae # Tonga moved from +12:20 to +13 in 1961, not 1941 https://github.com/eggert/tz/commit/666022ef2a65bb9872ffa6d98cb73236c5f6daa0 # Tonga switched to standard time in 1945, not 1901 https://github.com/eggert/tz/commit/03cc98977ea7c93c49ecb58eae07cd01e9034bfd # Cook Islands had two Christmases in 1899 https://github.com/eggert/tz/commit/bf12c3ba8513b640b83c245f804c37feeccb862b # More pre-1978 fixes for Cook and Niue https://github.com/eggert/tz/commit/a10451bb3fa7f84181f29ac81b06008fd1d81249 # Fix north Vietnam lat+long https://github.com/eggert/tz/commit/6860c875400b70be52c71d7b7b9642b8667a8112 # Niue left -11:20 in 1964, not 1978 https://github.com/eggert/tz/commit/b73f9fd0364e1795b40cd325661ce609d6126377 # Samoa no longer observes DST https://github.com/eggert/tz/commit/29583c461a9705118560e02e69bb64a0bf2fce0c # Jordan now starts DST on February’s last Thursday. https://github.com/eggert/tz/commit/39df8c8b22605f59f71213cfb92b3fd321e31d3c
These changes are in chronological order -- from old to new, so apply them accordingly.
“Replace Pacific/Enderbury with Pacific/Kanton” change was skipped as it requires changes in CLDR and tzdb2021b was announced late in CLDR's release cycle.
Code changes were skipped as Android uses fixed version of tzcode, not ToT.