You're right, the new Fair Usage Policy suggests that limits may be imposed especially when many requests are sent to the server. If that was your use case, I would suggest either setting up your own Export Server or using the offline exporting module.

The usage of offline exporting is very simple: https://jsfiddle.net/9cheza5d/

When the module is imported, all the charts get exported without an additional request to the export server by default (BUT not always, here's a chart showing when: https://assets.highcharts.com/images/client-side-export-flow.svg)

Another things worth keeping in mind:

1. You need to have the same version of Highcharts, exporting.js and offline-exporting.js:

<script src="https://code.highcharts.com/8.0.0/highcharts.js"></script>
<script src="https://code.highcharts.com/8.0.0/modules/exporting.js"></script>
<script src="https://code.highcharts.com/8.0.0/modules/offline-exporting.js"></script>

You can't mix different versions - I would suggest you use 11.4.8 for all the Highcharts packages that you import. That's most likely the culprit of the isSafari issue as I can't reproduce it with either v8 or v11. If you can reproduce it in JSFiddle, please share a link and I will investigate.

2. Regarding webgrease, I would suggest you clear the cache and re-bundle resources -- if your config is proper, it should help.

3. If you have some custom JS code in regards to offline exporting, you can use the exportChartLocal method documented here and here's a demo.