RCS seems to be replacing email-SMS gateways. RCS is supposedly an open standard, but apparently if you do not have Android 8.1 or newer, you cannot use RCS to send an SMS in Belgium.
In principle, iiuc, you should be able to use a desktop app or browser to send an RCS msg. But for some reason it’s a shitshow.
Can you elaborate on the limitations? In Belgium it seems more like a shitty deployment, but I don’t see what blocks a community FOSS option. Is it just that no one has been inspired to make an open source tool, or are the carrier’s APIs proprietary, secret, or restricted?