dropleft

bootstrap5.css

.dropup,
.dropright,
.dropdown,
.dropleft {
  position: relative;
}

bootstrap5.css

.dropleft .dropdown-menu {
  top: 0;
  right: 100%;
  left: auto;
  margin-top: 0;
  margin-right: 0.125rem;
}

bootstrap5.css

.dropleft .dropdown-toggle::after {
  display: inline-block;
  margin-left: 0.255em;
  vertical-align: 0.255em;
  content: "";
}

bootstrap5.css

.dropleft .dropdown-toggle::after {
  display: none;
}

bootstrap5.css

.dropleft .dropdown-toggle::before {
  display: inline-block;
  margin-right: 0.255em;
  vertical-align: 0.255em;
  content: "";
  border-top: 0.3em solid transparent;
  border-right: 0.3em solid;
  border-bottom: 0.3em solid transparent;
}

bootstrap5.css

.dropleft .dropdown-toggle:empty::after {
  margin-left: 0;
}

bootstrap5.css

.dropleft .dropdown-toggle::before {
  vertical-align: 0;
}

bootstrap5.css

.dropleft .dropdown-toggle-split::before {
  margin-right: 0;
}

html bootstrap5 Sample

左側表示のドロップメニュー：dropleft

  <div class="container py-5 d-flex justify-content-center">
    <!-- 左方向へのドロップ -->
    <div class="btn-group dropleft">
      <button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true"
        aria-expanded="false">左方向へのドロップ</button>
      <div class="dropdown-menu" aria-labelledby="dropdownMenuLink">
        <a class="dropdown-item" href="#">メニュー01</a>
        <a class="dropdown-item" href="#">メニュー02</a>
        <a class="dropdown-item" href="#">メニュー03</a>
      </div>
    </div>
  </div>