Wow I got pretty surprised a few days ago.
Thus, I saw that The R&A and USGA are planning to issue Clarifications to the Rules of Golf four times a year.
These Clarifications are not in the Rules themselves (Rule 1.3a) directly defined as “Rules”, but are supposed to be a kind of an explanation to things unclear in the Rules.
But…a few of the already issued Clarifications contains changes to the Rules of Golf. E.g. on April 9th a Clarification was issued giving the Club Committee an option to have a Local Rule in place allowing (under certain conditions) a player to replace a broken or substantially damaged club. This is (in my opinion) a change, since players did not have that option one week ago.
According to the USGA website there will normally be issued clarifications in January, April, July and October.
You can find all clarifications at this link.