The full warning says:
Using these device identifiers is not recommended other
than for high value fraud prevention and advanced telephony use-cases.
For advertising use-cases, use AdvertisingIdClient$Info#getId and for
analytics, use InstanceId#getId.More info:
https://developer.android.com/training/articles/user-data-ids.html
I think Android does not recommend using this value, since it’s a permanent unique id, which can be used to track your user, and he or she can’t change this id or prevent you to read it. The other options given in the warning respects your user’s wishes. That’s why Android recommend you to use that.
It really depends on what you are going to do with that id. Privacy is a sensitive thing these days.
Also check this:
…you must abide by a user’s ‘Opt out of interest-based advertising’
or ‘Opt out of Ads Personalization’ setting. If a user has enabled
this setting, you may not use the advertising identifier for creating
user profiles for advertising purposes or for targeting users with
personalized advertising. Allowed activities include contextual
advertising, frequency capping, conversion tracking, reporting and
security and fraud detection.
Source: https://developer.android.com/training/articles/user-data-ids.html