Is MacBook High Sierra 10.13.6 compatible with iOS 18?

My preliminary conclusion is no.

I connected my iPhone to an iMac running High Sierra and it did not appear anywhere. Not in the Finder, nor in the Photos app, nor iTunes. As far as that iMac is concerned the iPhone is simply not there. It appears in the USB device tree, and it charges, but that's it. Tried two different cables but only one Mac. I have another Mac that can boot High Sierra that I have not tried, but I doubt it will work either.

Oddly enough I was able to import the iPhone's photos using an even older iMac running El Capitan, but not the one running High Sierra. The reason may be that the older Mac did not receive the iPhone connection patch described at length here: "Your iPhone needs to update software to your Mac" - Apple Community but declining that offer is unlikely to be a suitable workaround in your case.

In your case I advise not upgrading iOS until you can upgrade your Mac. Support for High Sierra is fading fast.

I believe that theoretically, manual synchronization (including file transfer) should work … but in practice, it may or may not. People on these forums have been reporting mixed results since Apple came out with iOS 17. (Admittedly, there might be a self-selection effect here, where people are more likely to post if they have encountered a problem.)

An alternative means of transfer for some documents is simply to place them on iCloud Drive, Google Drive, or Microsoft OneDrive.

A Nac running High Sierra and an iPhone running iOS 18 can both access iCloud. You can also install the Google Drive and Microsoft OneDrive apps on the iPhone , in which case you can then access those cloud drives from the Files app.