We (ThinLinX) added RASClient 16.5 in our Debian-based thin client O/S - TLXOS - three years ago in our 4.10.0 (progressive) and 4.8.2 (LTS) releases, and have retained it ever since. At the time, we tested the 16.5.0 client (ARMHF, i386 and x86_64) versus a 19.0 RAS server and it still worked, but our server license expired some time ago and we're not certain if it still does - all that we can check is that the client still starts. Plus of course 16.5.0's feature set is outdated compared with that of the 19.x client.
We're currently considering upgrading RASClient to the latest available version (currently 19.3) in our latest x86_64-based editions (TLXOS SFF 5.0.x and TLXOS RePC 5.2.x); to date we have held these at the 16.5.0 level for consistency across all TLXOS editions.
It looks to me as if it would (still) be trivially simple to compile RASClient for other Linux architectures/ABIs, as there seems to be nothing in there that uses architecture-specific assembly code optimization; it relies on Qt to do all of the heavy lifting. If Parallels were to ressurect an ARM Linux port, ARM64 (aarch64) would probably be more useful than 32-bit ARM (ARMHF) these days.
By way of trivia: Unlike other commercial clients such as Citrix Receiver and VMware Horizon Client, RASClient 16.5.0 will run on ARMv6, i.e. Pi v1 and the original Pi Zero, where it actually performs surprisingly well, considering CPU power limitations. We therefore included it in our mostly defunct TLXOS RPi IoT (Rasbian-based) edition also.