Can I delete old iOS installer files on my macbook?
iOS installer files (IPSWs) can be safely removed.

IPSWs are not used as part of the backup or backup restore procedure, only for iOS restore, and as you can only restore signed IPSWs the older IPSWs can't be used anyway (without exploits).

You can always re-download IPSWs from Apple's site if you need them in the future.