Traditionally Apple Keyboard for Tamil has supported “Tamil Standard” [there are some anecdotal frustrations of senior Tamil computing innovators about Apple culturally appropriating this ‘Tamil99’ as their ‘standard’ and stripping away any mention of Tamil99, but this is for another day. Shh.] or Tamil99 layout which is quite useful for most practitioners. However one may observe in Tamil computer user community the prevalence of preference for transliteration input [Google has trained us by providing some good tools here, and Apple follows suit].
Beginning with iOS 12, Apple has released a Tamil transliterator. இதன் வழி எப்படியும் தமிழில் டைப்பிங் செய்யலாம்.

Apple iOS v12 update provides an additional IM by Tamil Keyboard with transliteration input. This July, 2018 updates provides this facility in Apple iPhone and Mac devices via iOS 12.
This method is useful for typing short sentences for my use so far, but I have to spend long enough time to pick it up. I hope Apple continues to support this feature and introduce Tamil input to several newer younger generation folks.
Plusses include – easy predictive input. Minusses include – somewhat non-intuitive mapping for certain letters. I cannot post a full review here, only for lack of trying. Thanks, Apple.