Can a 16-months-old child be placed in a bouncer to sleep under adequate supervision at all times?

A bouncinette does not provide a safe environment for any baby to sleep.
Red Nose recommends a baby is slept on a firm, flat (not elevated) surface for sleep.
https://rednose.org.au/article/bouncinette

Product Safety Australia (ACCC) has the following information about bouncinette safety

Safety tips

✓ Always place your baby in the bouncinette’s harness. 

✓ Put the bouncinette on a flat floor surface, away from potential hazards, such as stairs and furniture. 

✓ Stop using the bouncinette when your baby starts to roll. 

✘ Never place the bouncinette on a table or other raised surface that could cause your baby to fall.

✘ Never carry your baby around in the bouncinette in case you trip or fall. 

✘ Never let your baby fall asleep in a bouncinette—they can slip down and strangle on the waist strap

https://www.productsafety.gov.au/system/files/Keeping baby Safe_0.pdf

Most child safety organizations recommend that bouncers are not used once baby is able to roll (usually 4-5mths)

If you have a bouncinette or baby chair, use it only on the floor, not on a table or raised surface. If your baby can roll over, don’t use a bouncinette.

https://raisingchildren.net.au/newborns/safety/equipment-furniture/safe-baby-furniture