If you use certbot-auto to install the client, part of the installation process involves compiling some crypto library (IIRC) which tends to fail on low-memory

If you use certbot-auto to install the client, part of the installation process involves compiling some crypto library (IIRC) which tends to fail on low-memory systems, hence the need for swap. Packaged versions of certbot (from your distribution's package manager) don't suffer from that problem and have fairly low memory requirements.