v:2 indexes were introduced in SERVER-25156 (in 2016 for MongoDB version 3.4). The v:2 index format became the default for new index builds starting in 3.4. Therefore, any v:1 indexes remaining on the Atlas fleet or in on-prem deployments in the wild were likely first constructed almost a decode ago and never dropped and rebuilt.

We hypothesize (but should double-check by examining telemetry) that this old index format has become quite rare in practice. There is no practical reason to continue using v:1 for new index builds. Now is a good time for us to do the following:

• Mark v:1 indexes as deprecated in the code base and our documentation.
• Prohibit the construction of new v:1 builds (being careful to fully support pre-existing v:1 index builds, including replication of existing v:1 indexes).
• Make sure that we have sufficient telemetry to allow us to proactively notify customers that have v:1 indexes so that we can encourage them to reindex as v:2. This is in preparation for a future effort to remove support for v:1.