chore: deprecate cxx14

[Becheler]

Deprecate C++14 in 1.93. C++17 planned for 1.95.

Important

New algorithms and API additions need a design issue first. Such a PR is not reviewed until an issue has settled the API and frozen the scope. PRs whose scope grows under review go stale. See How To Contribute an Algorithm.

Motivation

See #613

What this PR does

Boost.Graph has no master header, unlike Boost.Geometry, so there is no single place every user already includes. This PR therefore adds detail/deprecate_cxx14.hpp, mirroring the Boost.Geometry deprecation PR, and includes it from graph_traits.hpp. That header is the library's most widely included one, directly by 83 of the 145 public headers and transitively by every algorithm entry point, so the notice should reach users whichever header they include.

Before submitting

• I searched for an existing PR or issue covering the same change.
• This PR targets the develop branch.
• My contribution is licensed under the Boost Software License 1.0.

Kind of change

Tick exactly one. Each kind is reviewed differently, so mixing them in one PR makes the diff difficult to review (why).

• Reformat (layout only, no token added or removed)
• Refactor (no observable behavior change)
• Behavior change (observable to users of the library)
• No code change (documentation, build, CI, or tooling)

Change category

• Bug fix
• New algorithm or API addition
• Deprecation or removal
• Performance
• Tests
• Documentation
• Build, CI, or tooling
• Other (specify below)

[cppalliance-bot]

An automated preview of the documentation is available at https://645.graph.prtest3.cppalliance.org/libs/graph/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-10-12 15:14:26 UTC

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 38267966442 vs develop run 38230475111 (f27cb02df3).

Header-inclusion weights (graph files pulling each direct dependency in):

Dependency	develop	PR	Δ
config	93	94	+1

Transitive Boost modules: 47 → 47 (0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 38267966453 vs develop run 38230475092 (f27cb02df3).

Job	Baseline	After	Delta
macos (clang, 14)	253	253	0
macos (clang, 17)	252	252	0
macos (clang, 20)	252	252	0
ubuntu (clang-19, 14)	253	253	0
ubuntu (clang-19, 17)	252	252	0
ubuntu (clang-19, 20)	252	252	0
ubuntu (clang-19, 23)	252	252	0
ubuntu (gcc-14, 14)	349	349	0
ubuntu (gcc-14, 17)	345	345	0
ubuntu (gcc-14, 20)	345	345	0
ubuntu (gcc-14, 23)	345	345	0
windows_msvc_14_3 (msvc-14.3)	937	937	0