[modified]

Apologies for cutting too much of the log. I'm running the official 6.26.0 on macOS.

I dug around and it looks like it's because calibre is pinned to mechanize v0.4.7 but support for HTTP308 redirects is available only from v0.4.8 (commit).

$ ebook-convert 'http308.recipe' .epub --test --debug-pipeline debug -vv
Conversion options changed from defaults:
  test: (2, 2)
  debug_pipeline: 'debug'
  verbose: 2
Resolved conversion options
calibre version: 6.26.0
{'asciiize': False,
 'author_sort': None,
 'authors': None,
 'base_font_size': 0,
 'book_producer': None,
 'change_justification': 'original',
 'chapter': None,
 'chapter_mark': 'pagebreak',
 'comments': None,
 'cover': None,
 'debug_pipeline': 'debug',
 'dehyphenate': True,
 'delete_blank_paragraphs': True,
 'disable_font_rescaling': False,
 'dont_download_recipe': False,
 'dont_split_on_page_breaks': True,
 'duplicate_links_in_toc': False,
 'embed_all_fonts': False,
 'embed_font_family': None,
 'enable_heuristics': False,
 'epub_flatten': False,
 'epub_inline_toc': False,
 'epub_max_image_size': 'none',
 'epub_toc_at_end': False,
 'epub_version': '2',
 'expand_css': False,
 'extra_css': None,
 'extract_to': None,
 'filter_css': None,
 'fix_indents': True,
 'flow_size': 260,
 'font_size_mapping': None,
 'format_scene_breaks': True,
 'html_unwrap_factor': 0.4,
 'input_encoding': None,
 'input_profile': <calibre.customize.profiles.InputProfile object at 0x118f1fbe0>,
 'insert_blank_line': False,
 'insert_blank_line_size': 0.5,
 'insert_metadata': False,
 'isbn': None,
 'italicize_common_cases': True,
 'keep_ligatures': False,
 'language': None,
 'level1_toc': None,
 'level2_toc': None,
 'level3_toc': None,
 'line_height': 0,
 'linearize_tables': False,
 'lrf': False,
 'margin_bottom': 5.0,
 'margin_left': 5.0,
 'margin_right': 5.0,
 'margin_top': 5.0,
 'markup_chapter_headings': True,
 'max_toc_links': 50,
 'minimum_line_height': 120.0,
 'no_chapters_in_toc': False,
 'no_default_epub_cover': False,
 'no_inline_navbars': False,
 'no_svg_cover': False,
 'output_profile': <calibre.customize.profiles.OutputProfile object at 0x118f1ead0>,
 'page_breaks_before': None,
 'prefer_metadata_cover': False,
 'preserve_cover_aspect_ratio': False,
 'pretty_print': True,
 'pubdate': None,
 'publisher': None,
 'rating': None,
 'read_metadata_from_opf': None,
 'remove_fake_margins': True,
 'remove_first_image': False,
 'remove_paragraph_spacing': False,
 'remove_paragraph_spacing_indent_size': 1.5,
 'renumber_headings': True,
 'replace_scene_breaks': '',
 'search_replace': None,
 'series': None,
 'series_index': None,
 'smarten_punctuation': False,
 'sr1_replace': '',
 'sr1_search': '',
 'sr2_replace': '',
 'sr2_search': '',
 'sr3_replace': '',
 'sr3_search': '',
 'start_reading_at': None,
 'subset_embedded_fonts': False,
 'tags': None,
 'test': (2, 2),
 'timestamp': None,
 'title': None,
 'title_sort': None,
 'toc_filter': None,
 'toc_threshold': 6,
 'toc_title': None,
 'transform_css_rules': None,
 'transform_html_rules': None,
 'unsmarten_punctuation': False,
 'unwrap_lines': True,
 'use_auto_toc': False,
 'verbose': 2}
1% Converting input to HTML...
InputFormatPlugin: Recipe Input running
Using custom recipe
Using user agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36
1% Fetching feeds...
1% Got feeds from index page
1% Trying to download cover...
1% Generating masthead...
Synthesizing mastheadImage
1% Starting download [4 threads]...
Fetching https://www.wsj.com/articles/mccarthy-biden-impeachment-inquiry-b9cc6530
Could not fetch link https://www.wsj.com/articles/mccarthy-biden-impeachment-inquiry-b9cc6530
Traceback (most recent call last):
  File "calibre/web/fetch/simple.py", line 278, in fetch_url
  File "mechanize/_mechanize.py", line 241, in open_novisit
  File "mechanize/_mechanize.py", line 313, in _mech_open
mechanize._response.get_seek_wrapper_class.<locals>.httperror_seek_wrapper: HTTP Error 308: Permanent Redirect

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "calibre/web/fetch/simple.py", line 536, in process_links
  File "calibre/web/fetch/simple.py", line 283, in fetch_url
calibre.web.fetch.simple.FetchError: Permanent Redirect

https://www.wsj.com/articles/mccarthy-biden-impeachment-inquiry-b9cc6530 saved to 
Failed to download article: HTTP 308 Direct from https://www.wsj.com/articles/mccarthy-biden-impeachment-inquiry-b9cc6530
Traceback (most recent call last):
  File "calibre/utils/threadpool.py", line 99, in run
  File "calibre/web/feeds/news.py", line 1195, in fetch_article
  File "calibre/web/feeds/news.py", line 1190, in _fetch_article
Exception: Could not fetch article. The debug traceback is available earlier in this log