Running phpcs --standard=Drupal,DrupalPractice --extensions=php,module,inc,install,test,profile,theme,css,info,txt,md,yml,twig shows these warnings/errors.
FILE: ./prefetch_cache_8.x-1.x/config/install/prefetch_cache.settings.yml
1 | ERROR | [x] Expected 1 newline at end of file; 0 found
FILE: ./prefetch_cache_8.x-1.x/CHANGELOG.txt
17 | ERROR | [x] Expected 1 newline at end of file; 0 found
FILE: ./prefetch_cache_8.x-1.x/src/RequestPolicy/GetRequest.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
16 | ERROR | [x] Additional blank lines found at end of doc comment
27 | ERROR | [x] Expected 1 blank line after function; 0 found
28 | ERROR | [x] The closing brace for the class must have an empty line before it
FILE: ./prefetch_cache_8.x-1.x/src/RequestPolicy/NoSessionUser.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
16 | ERROR | [x] Additional blank lines found at end of doc comment
27 | ERROR | [x] Expected 1 blank line after function; 0 found
28 | ERROR | [x] The closing brace for the class must have an empty line before it
FILE: ./prefetch_cache_8.x-1.x/src/Cache/ChainCacheTagsGeneratorInterface.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
15 | ERROR | [x] Additional blank lines found at end of doc comment
21 | ERROR | [ ] Missing parameter comment
31 | ERROR | [x] Expected "\Drupal\prefetch_cache\Cache\CacheTagsGeneratorInterfacephp" but found
| | "\Drupal\prefetch_cache\Cache\CacheTagsGeneratorInterface.php" for parameter type
37 | ERROR | [x] Expected 1 newline at end of file; 0 found
37 | ERROR | [x] The closing brace for the interface must have an empty line before it
FILE: ./prefetch_cache_8.x-1.x/src/Cache/CacheTagsGeneratorInterface.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
15 | ERROR | [x] Additional blank lines found at end of doc comment
FILE: ./prefetch_cache_8.x-1.x/src/Cache/ChainCacheTagsGenerator.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
16 | ERROR | [x] Additional blank lines found at end of doc comment
27 | ERROR | [x] @inheritdoc found. Did you mean {@inheritdoc}?
43 | ERROR | [x] @inheritdoc found. Did you mean {@inheritdoc}?
FILE: ./prefetch_cache_8.x-1.x/src/StackMiddleware/PrefetchCacheBeforePageCache.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
25 | ERROR | [x] Additional blank lines found at end of doc comment
31 | ERROR | [x] Expected "\Drupal\prefetch_cache\Cache\ChainCacheTagsGeneratorInterface" but found
| | "\Drupal\prefetch_cache\Cache\ChainCacheTagsGeneratorInterface." for @var tag in member variable comment
81 | WARNING | [x] There must be no blank line following an inline comment
81 | WARNING | [ ] There must be no blank line following an inline comment
125 | ERROR | [x] Expected 1 space before opening brace; found 2
139 | WARNING | [ ] \Drupal calls should be avoided in classes, use dependency injection instead
145 | ERROR | [ ] Missing parameter comment
145 | ERROR | [ ] Missing parameter type
146 | ERROR | [ ] Missing parameter comment
146 | ERROR | [ ] Missing parameter type
147 | ERROR | [x] Separate the @param and @return sections by a blank line.
147 | ERROR | [ ] Description for the @return value is missing
160 | ERROR | [ ] Missing parameter comment
161 | ERROR | [ ] Missing parameter comment
161 | ERROR | [ ] Missing parameter type
162 | ERROR | [ ] Missing parameter comment
162 | ERROR | [ ] Missing parameter type
176 | ERROR | [x] Expected 1 blank line after function; 0 found
177 | ERROR | [x] The closing brace for the class must have an empty line before it
FILE: ./prefetch_cache_8.x-1.x/src/StackMiddleware/PrefetchCacheAfterSessionInitialized.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
21 | ERROR | [x] Additional blank lines found at end of doc comment
24 | ERROR | [x] Missing function doc comment
70 | ERROR | [x] Expected 1 space before opening brace; found 2
FILE: ./prefetch_cache_8.x-1.x/src/PrefetchCacheInterface.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
18 | ERROR | [x] Additional blank lines found at end of doc comment
40 | ERROR | [x] Expected 1 newline at end of file; 0 found
40 | ERROR | [x] The closing brace for the interface must have an empty line before it
FILE: ./prefetch_cache_8.x-1.x/src/ResponsePolicy/CacheOnlyPrefetchCacheRequests.php
3 | ERROR | [x] Namespaced classes, interfaces and traits should not begin with a file doc comment
FILE: ./prefetch_cache_8.x-1.x/prefetch_cache.module
1 | ERROR | [x] Missing file doc comment
8 | ERROR | [x] Expected 1 newline at end of file; 0 found
FILE: ./prefetch_cache_8.x-1.x/prefetch_cache.libraries.yml
8 | ERROR | [x] Expected 1 newline at end of file; 0 found
FILE: ./prefetch_cache_8.x-1.x/README.md
1 | WARNING | Line exceeds 80 characters; contains 127 characters
3 | WARNING | Line exceeds 80 characters; contains 86 characters
4 | WARNING | Line exceeds 80 characters; contains 239 characters
Those warnings/errors must be fixed, when possible.