What is the latest supported SDK version for FR45 and Garmin Swim

I assume you mean FR45 and Garmin Swim 2, as I don't think Garmin Swim supports CIQ.

The connectIQVersion under {DEVICE_ID}/compiler.json:partNumbers[] is the max CIQ version (as far as the SDK is concerned), not the min SDK version. In the case of those two watches, it's 1.4.5.

You'll notice if you edit your watchface project so that the min SDK is greater than 1.4.0, you can't build for those products.

Having said that, sometimes there is a lag between real-world firmware support for CIQ and support in the SDK's device library, especially for Asia-Pacific (APAC) models.

But In the case of FR45 and Garmin Swim 2, I think the max CIQ version will be CIQ 1.4.5 forever.

Here's why:

FR45 and Garmin Swim 2 are unique amongst CIQ devices in that:

1) They support only watchface CIQ apps and no others (data fields, widgets or device apps)

2) They have CIQ 1.4.x but they were released fairly recently (2 years ago). (Most other CIQ 1 devices are about 5 years old)

3) They have a connectIQVersion of 1.4.5. (You'll notice every other other CIQ 1 device has a connectIQVersion of 1.4.4). I would say that CIQ 1.4.5 was probably created just for these watches.

Also:

4) Every old device that currently has connectIQVersion of 1.x.x is known to be stuck on CIQ 1 forever, due to resource limitations.

5) IIRC, no CIQ 1 device ever received CIQ 2 (because of limited hardware resources -- CIQ 2 devices all have twice the memory for datafields as CIQ 1, for example)

Based on all of this, I'd say that FR45 and Garmin Swim 2 are intentionally limited so that they only run watchfaces on an old version of CIQ (probably due to intentional hardware limitations, to save money and segment the market), and it's unlikely that they'll ever be updated past CIQ 1.4.

Another thing to note is with the fr45/Swim2, Watch faces are limited to 48k, where other CIQ 1 devices, they have a max of 64k.

So it's not only just watch faces, it's simple watchfaces.