UART isolator using TI ISO6721.  TI promises 50 Mbit/s data rate, so it can be used for many other use cases as well.  This design is by a hobbyist, and assumes a few Mbit/s data rates at maximum.  The first version of the board is slightly under 18×18mm, with two 3mm and four 2mm mounting holes; the second version is as shown in the images here with no mounting holes, size 13×11mm.

Each end supplies their own VCC, which defines the signal levels at that end only.  Suitable voltages are 1.71 V – 1.89 V, or 2.25 V – 5.5 V.  (That is, VCC between 1.90 V and 2.24 V is not supported.)  This means that not only does this provide isolation (from e.g. ground loops), it also allows for voltage level shifting, with either end at for example 1.8 V, 2.4 V, 2.8 V, 3.0 V, 3.3 V, 3.6 V, 3.8 V, 4.0 V, 4.4 V, 4.8 V, 5.0 V, or 5.25 V.  The exact current draw is minimal, a few milliamps total.

BOM:

• TI ISO6721FBQDRQ1.  This defaults to outputs low, which in UART use lets the other endpoint detect when the other end is powered on.
• Two 100nF/0.1µF to 1µF ceramic capacitors in 0603 or 0805 footprint.  I recommend using X7R with at least 16V voltage rating here.