First, you omitted the OTF features we request for Tamil -- aren't
they needed for the Supplement block? if not, why not?
Tamil supplement does not need the akhn OTF feature because it does not form any conjuncts, they are mostly accounting or agricultural symbols.
Next, we should consider what the main supported platforms do with
this block -- don't they come with other fonts configured for this
block? if they do, we should at least use 'append', not 'prepend'.
There does not seem to be any font in MS windows which supports these characters.
I am not sure about MacOS.
but still I will use append.
And finally, is the license of Noto Sans Tamil Supplement free enough
for us to include it in our sources?
The Noto Sans Tamil Supplement font file is under the SIL Open Font License v1.1
The Noto Color Emoji font is also under the same license and is referenced in the Emacs source code.