phpseclib/phpseclib suggests installing ext-libsodium (SSH2/SFTP can make use of some algorithms provided by the libsodium-php extension.)
phpseclib/phpseclib suggests installing ext-mcrypt (Install the Mcrypt extension in order to speed up a few other cryptographic operations.)
phpseclib/phpseclib suggests installing ext-gmp (Install the GMP (GNU Multiple Precision) extension in order to speed up arbitrary precision integer arithmetic operations.)
google/apiclient suggests installing cache/filesystem-adapter (For caching certs and tokens (using Google_Client::setCache))
bacon/bacon-qr-code suggests installing ext-imagick (to generate QR code images)
intervention/image suggests installing ext-imagick (to use Imagick based image processing.)
intervention/image suggests installing intervention/imagecache (Caching extension for the Intervention Image library)
aws/aws-sdk-php suggests installing ext-sockets (To use client-side monitoring)
aws/aws-sdk-php suggests installing aws/aws-php-sns-message-validator (To validate incoming SNS notifications)
doctrine/cache suggests installing alcaeus/mongo-php-adapter (Required to use legacy MongoDB driver)