Is IMAP CONDSTORE widely supported?

As I needed a more precise answer, I did a small node.js script and used some test accounts.

Here are the results, if you need more accounts, tell me.

yahoo

No CONDSTORE.

• IMAP4REV1
• ID
• NAMESPACE
• X-ID-ACLID
• UIDPLUS
• LITERAL+
• XAPPLEPUSHSERVICE
• XYMHIGHESTMODSEQ

gmail

CONDSTORE supported.

• IMAP4REV1
• UNSELECT
• IDLE
• NAMESPACE
• QUOTA
• ID
• XLIST
• CHILDREN
• X-GM-EXT-1
• UIDPLUS
• COMPRESS=DEFLATE
• ENABLE
• MOVE
• CONDSTORE
• ESEARCH

outlook.com

No CONDSTORE.

• IMAP4REV1
• CHILDREN
• ID
• NAMESPACE
• UIDPLUS
• UNSELECT

No, it is not widely supported. You can treat it as an optimization to use when available, but you cannot depend on it. You also need a way to track removed messages, which is not something condstore can tell you

Gmail, however, just rolled out support for it recently.