According to ThomasV, they do not accept any form of donations, and the argument is that whoever accepts money is not interested in future development.
Thank you for your support. I usually do not request donations for myself, because I do not think it is an effective way to incentivise free software.
Developers who request bounties tend to finish projects quickly, and do not really care about long-term maintenance.
However, you you can donate to the people who run public Electrum servers.
In general, the use of the wallet is free (it is open source,) but it is not free because whoever operates Electrum servers can sell your data, so you may need some costs to run your own server.