I don't know of a nice way to solve this but I ran into the same problem. A package I've never heard of was warning that it was abandoned. My solution was to search the composer.lock file for the abandoned package name. It will appear in require or require-dev for the package that depends on it.
In my case it was several levels, package A depended on package B that depended on abandoned package C. Once I identified what package A was then composer show --tree package/a showed the abandoned package in the tree output
When you have the package name of a deep dependent, and you want to know to what root dependent it belongs, use composer depends.
$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 0 updates, 0 removals
Package guzzle/guzzle is abandoned, you should avoid using it. Use guzzlehttp/guzzle instead.
Writing lock file
Generating autoload files
$ composer depends guzzle/guzzle
aws/aws-sdk-php 2.8.31 requires guzzle/guzzle (~3.7)
Your comment on another answer suggested you were trying to untangle a dependency problem. Here's an example using depends to do that:
$ composer require phan/phan
Using version ^1.1 for phan/phan
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.
Problem 1
- Installation request for composer/xdebug-handler (locked at 1.1.0) -> satisfiable by composer/xdebug-handler[1.1.0].
- phan/phan 1.1.0 requires composer/xdebug-handler ^1.3 -> satisfiable by composer/xdebug-handler[1.3.0].
- phan/phan 1.1.1 requires composer/xdebug-handler ^1.3 -> satisfiable by composer/xdebug-handler[1.3.0].
- phan/phan 1.1.2 requires composer/xdebug-handler ^1.3 -> satisfiable by composer/xdebug-handler[1.3.0].
- phan/phan 1.1.3 requires composer/xdebug-handler ^1.3 -> satisfiable by composer/xdebug-handler[1.3.0].
- phan/phan 1.1.4 requires composer/xdebug-handler ^1.3 -> satisfiable by composer/xdebug-handler[1.3.0].
- Conclusion: don't install composer/xdebug-handler 1.3.0
- Installation request for phan/phan ^1.1 -> satisfiable by phan/phan[1.1.0, 1.1.1, 1.1.2, 1.1.3, 1.1.4].
Installation failed, reverting ./composer.json to its original content.
$ composer depends composer/xdebug-handler
friendsofphp/php-cs-fixer v2.12.1 requires composer/xdebug-handler (^1.0)
So, I wanted phan/phan, but that failed because of a version problem on composer/xdebug-handler, which is not a package I've ever asked for explicitly.
Then I ask what packages "depend" on composer/xdebug-handler and discover that friendsofphp/php-cs-fixer needs it (and I know about that package, it's a root dependent).
Then I note that phan/phan wants composer/xdebug-handler:^1.3and (from the depends) that friendsofphp/php-cs-fixerallows me to have version 1.3. So now I just do an update:
$ composer update composer/xdebug-handler
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 1 update, 0 removals
- Updating composer/xdebug-handler (1.1.0 => 1.3.0): Loading from cache
Writing lock file
Generating autoload files
$ composer require phan/phan
Using version ^1.1 for phan/phan
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 5 installs, 0 updates, 0 removals
- Installing sabre/event (5.0.3): Loading from cache
- Installing microsoft/tolerant-php-parser (v0.0.15): Loading from cache
- Installing netresearch/jsonmapper (v1.4.0): Loading from cache
- Installing felixfbecker/advanced-json-rpc (v3.0.3): Loading from cache
- Installing phan/phan (1.1.4): Loading from cache
phan/phan suggests installing ext-ast (Needed for parsing ASTs (unless --use-fallback-parser is used). php-ast ^0.1.5|^1.0.0 is needed.)
Writing lock file
Generating autoload files
The question has already been answered, but Composer offers, in my opinion, a more eloquent way, which hasn't been mentioned before: depends command alias why.
composer why aims to answer the "Why is this package installed?" question, instead of "Which packages depend on this package?", which I find much easier to remember.
Being an alias, the why command behaves the same as depends and both aforementioned options still apply:
--recursive (-r): Recursively resolves up to the root package;
--tree (-t): Prints the results as a nested tree, implies -r.