Are extensions that work with CiviCRM 4.7 also compatible with 5.x?
Because 5.0 does not include breaking changes, it is considered equally compatible with 4.7 extensions. There is always minor risk of breakage when upgrading software, but 4.7 to 5.0 is not considered a major upgrade.

From https://lab.civicrm.org/dev/release/issues/1#note_2843


  
  In the 4.x series,  tags mean the same as today. (Exact-match to version. An extension declaring 4.6 is not displayed for
  download on 4.7.)
  In the 5.x series,  tags imply forward compatibility. (An extension declaring 5.2 is displayed on 5.2, 5.3, etc. But
  not on 5.0.)
  To bridge them, 4.7 is forward compatible with 5.0. (An extension declaring 4.7 is displayed on 4.7, 5.0, 5.1, etc.)
  As today, a consumer sees the newest extensions that are compatible with their version.