-able vs -eable is not about GB or US spelling

The grammar rule is that the final silent "e" of the root verb is
dropped, except for root verbs ending in "ce" and "ge". The rule is
the same in British and American English, and so is actual usage
according to the Google Ngram Viewer:
https://books.google.com/ngrams/graph?content=writable%2Cwriteable&year_start=1900&year_end=2019&corpus=en-GB-2019
https://books.google.com/ngrams/graph?content=writable%2Cwriteable&year_start=1900&year_end=2019&corpus=en-US-2019

codespell_lib/data/dictionary.txt

@@ -60860,7 +60860,7 @@ wireframws->wireframes
 wirh->with
 wirhin->within
 wirhout->without
-wirtable->writable, writeable,
+wirtable->writable
 wirte->write
 wirter->writer
 wirters->writers
@@ -60881,7 +60881,7 @@ wissle->whistle
 wissled->whistled
 wissles->whistles
 wissling->whistling
-witable->writable, writeable,
+witable->writable
 witdh->width
 witdhs->widths
 witdth->width

codespell_lib/data/dictionary_en-GB_to_en-US.txt

@@ -518,4 +518,3 @@ visualised->visualized
 visualiser->visualizer
 visualises->visualizes
 visualising->visualizing
-writeable->writable