Upon checking out the geoservices page, I believe you are referring to the documentation regarding Updated TileMatrixSet for ORTHOIMAGERY.ORTHOPHOTOS layer in which the max zoom level was changed to 19 which was previously set at 21.

I managed to reproduce the issue you described where 400 errors were returned for tile requests beyond the native max, and when those network errors were bypassed:

• 404/400 Network Error: https://jsfiddle.net/btvqrun3/
• No Network Error but null tiles (blank): https://jsfiddle.net/92gwcyqa/

In regards to your question:

"How can I stretch or interpolate the last available zoom level (19) to simulate higher zoom levels (20, 21, 22) in Google Maps?"

You could try a workaround for this by applying over-zoom functionality via cropping & scaling the parent tile in your map which enables you to zoom past level 19 and even more than 22. Instead of requesting non-existent tiles at z>19, you can fetch the parent tile at z=19 and crop + scale the correct quadrant to fill each child tile at z=20–22.

Here’s a minimal, reproducible sample (uses IGN WMTS as in your post):

• Workaround (Crop & Scale): https://jsfiddle.net/2podsux0/