They should just build a mobile app for the purpose of receiving these notifications. Make the app $2.99. Turn the expense into a profit. /s
Pushover is $5 once for personal use, ntfy.sh can be completely self-hostable if you prefer.
I have written a small tool which utilizes pushover for these reasons.
You can receive the notifications on your browser/mobile for free afterwards.
So it's even work if you don't have control about the le client.
And worse of all, worry about Apple and Google's arbitrary rejections?
This seems far more costly than email. Just having one dev keeping those apps going, is likely 20x or more than their email costs per year.