there is no change visible with zdump
That's true, but zdump (contrary to its name) doesn't just dump the content of a TZif file.
This is the 2023c/2023d difference I see in the raw, slim Europe/London data, just as Derick reports.
@@ -176,6 +176,6 @@
782874000 [type=2] gmtoff= 0 is_dst=F isstd=0 isgmt=0 abbr="GMT"
796179600 [type=1] gmtoff= 3600 is_dst=T isstd=0 isgmt=0 abbr="BST"
814323600 [type=2] gmtoff= 0 is_dst=F isstd=0 isgmt=0 abbr="GMT"
- 828234000 [type=1] gmtoff= 3600 is_dst=T isstd=0 isgmt=0 abbr="BST"
+ 820454400 [type=2] gmtoff= 0 is_dst=F isstd=0 isgmt=0 abbr="GMT"
Future Specification:
GMT0BST,M3.5.0/1,M10.5.0