I’ve installed 100s directly over shingles. It doesn’t void the warranty that is a myth. Lath strips are good if that’s what you like. I’ve had to go replace metal after trees landed on houses and never seen rust. You can unscrew any metal job I’ve ever done never seen any condensation under the metal. Another thing some people do is put titanium under layment over the shingles. Pm me if you need any help

$600 difference in shingles vs 26 ga metal on mine.
The metal is going to last much longer. Ever seen a 50+ yr old shingle roof? There are a ton of 50+ yr old barns with galvanized metal still doing its thing.
Im installing 2x4 batten strips under mine. The house is 40+ yrs old and the strips will eliminate amd defects and ensure a flat level roof.

The answer to the question of can you directly apply a metal panel roof directly on an existing shingle roof is this: Maybe.

Metal roofing manufacturers have specific installation instructions. Some allow and some don't. Some allow with certain additional details such as an underlayment over the shingles (slip sheet to reduce friction from differential expansion and contraction) ...and/or furring strips so there is an airspace between the new metal and the existing shingles. Always follow the specific instructions from the metal manufacturer. Period.

Best practices are underlayment and furring strips. Key word is "best". There are ways to do things and then there are the best ways to do things.

Local building codes and officials may have certain requirements for their municipality or jurisdiction.

If your existing decking has issues then that would need to be resolved before new metal installation.

So...maybe. It truly depends on those factors. I investigate construction defects, deficiencies, water intrusion and building envelope components for a living. 80% of my work is defending, when possible, builders, subcontractors and roofers.