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Google Slides Web Alternative...
(lemmy.zip)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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If you don't mind clutter free, simple presentations, try hack.md or cryptpad. Both supports markdown styled presentations, both are FOSS cloud applications and the format of the presentation is highly portable.
If you want to get really ambitious, try marp framework for presentations - still markdown but on steroids.
I haven't used Cryptpad's slides feature but I can vouch for it's Google Docs alternative. It came in clutch late in college when I ran out of Google cloud storage!