Why is Java 10 recommended if you're using the G1 GC?

Because it wasn't until Java 10 that G1GC became fully parallel in stop-the-world full GC cycle. As per JEP 307: Parallel Full GC for G1 this improves the latency of the worst case scenario:

The G1 garbage collector is designed to avoid full collections, but when the concurrent collections can't reclaim memory fast enough a fall back full GC will occur. The current implementation of the full GC for G1 uses a single threaded mark-sweep-compact algorithm. We intend to parallelize the mark-sweep-compact algorithm and use the same number of threads as the Young and Mixed collections do. The number of threads can be controlled by the -XX:ParallelGCThreads option, but this will also affect the number of threads used for Young and Mixed collections.