Supposably is a synonym of purportedly.
As adverbs the difference between purportedly and supposably
is that purportedly is supposedly, putatively, or reputedly while supposably is able to be supposed or conjectured.
purportedly
English
Adverb
(-)
supposedly, putatively or reputedly
supposably
English
Adverb
(-)
Able to be supposed or conjectured.
(nonstandard) Supposedly.
Synonyms
* (able to be supposed) conceivably, imaginably
* (supposedly) purportedly